Initial Permeability of Co-Based Amorphous Films at High Frequencies

Frequency characteristics of the initial permeability (¿i\´ and ¿i") of Co-based amorphous films (Co-Ta-Hf, Co-(Nb, Ta)-Zr) at high frequencies (400 kHz to 80 MHz) were investigated. Two factors influencing the initial permeability at high frequencies, natural resonance and eddy current effects, are discussed. It was found that the relation of the conventional "Snoek\´s limit" to the high-frequency permeability applies in these Co-based amorphous films if the film thickness and uniaxial anisotropy are small enough, and if the effect of the demagnetization field is negligible. The induced large unidirectional anisotropy improves the initial permeability along the hard magnetic axis in the high frequency range; this is caused by changes in the initial magnetization process and rises in the natural resonance frequency. The influence of eddy currents on the degradation of the initial permeability becomes a dominant factor at high frequencies in proportion to the film thickness, because of the metallic conductivity of the film.